1) Field of the Invention
The present invention relates to a technology for an image forming apparatus.
2) Description of the Related Art
In a conventional image forming apparatus, there are a couple of types of rotating belts, including an intermediate transfer belt used in an intermediate transfer system and a transfer sheet conveying belt used to support and carry the transfer sheet and let the transfer sheet pass through a transfer nip.
An example of an image forming apparatus that uses an intermediate transfer belt is, for instance, an image forming apparatus of a tandem type intermediate transfer system. The image forming apparatus of the tandem type intermediate transfer system includes a plurality of photosensitive drums, each of which has an individual developing device. A single color toner image is formed on each photosensitive drum and each color toner image is sequentially copied on to the intermediate transfer belt. Consequently, a combined color image is formed on the intermediate transfer belt, and the combined color image on the intermediate transfer belt is then batch-transferred on to the transfer sheet.
An example of an image forming apparatus that uses a transfer sheet conveying belt is, for instance, an image forming apparatus of a tandem type direct transfer system. The image forming apparatus of a tandem type direct transfer system includes a plurality of photosensitive drums, each of which has an individual developing device. A single color toner image is formed on each photosensitive drum, and each color toner image is sequentially superimposed on to a single transfer sheet, which is supported and carried by the transfer sheet conveying belt, and consequently a combined color image is formed on the transfer sheet.
The rotating belt (hereinafter, ‘belt’) is usually suspended by a plurality of rollers. A driving roller from among the rollers drives the belt in an endless movement.
Conventionally, a material is attached on one portion of the front or the back in the width direction of the belt for various purposes, such as a sheet used as a scale (hereinafter, ‘scale’) to detect and read an amount of movement of the belt by means of a sensor (see, for example, Japanese Patent Laid-Open Publication No. H11-24507) or a protection seal that is attached on both edges of the belt to prevent cracks being developed at the edges of the belt. At the time of driving the belt, in order to prevent biasing of the belt towards one end of the axis direction of a roller, a stopper is also attached in such a way that it protrudes within both edges of the belt. FIG. 13 is a schematic diagram of a belt V provided in an image forming apparatus. At both edges of the belt in the width direction, a material (hereinafter, ‘affixed material S’) is attached. When the belt V is driven in the image forming apparatus, it rotates in direction α, as shown in FIG. 13.
In the image forming apparatus that employs the belt V, in which the affixed material S is attached to a portion on the front or the back of the belt, there is a chance to occur a problem as follows.
When the belt V continues to rotate, both edges of the belt in the width direction are about to bend inwards. FIG. 14 is a cross section of the belt cut along X in FIG. 13, illustrating that the edges of the belt are bent inwards. FIG. 15 is a magnified view of the end of the belt encircled in FIG. 14. As shown in FIG. 14, when the affixed material S is provided at both edges in the width direction on the inner surface of the belt, both ends of the belt bend towards the side of the affixed material S. If the belt V is suspended by a plurality of rollers and a tension is applied, a uniform tension is applied in the width direction in the belt V. However, an amount of stretch at the edges of the belt, where the affixed material S is provided, is smaller than an amount of stretch at the center part of the belt, where the affixed material S is not provided. Due to a difference in the amount of stretch, a peripheral length of the part where the amount of stretch is smaller and the affixed material S is provided is shorter than the peripheral length of the part where the amount of stretch is greater. As a result, as shown in FIG. 15, a deformation appears at a boundary d between the portion where the amount of stretch is smaller and the portion where the amount of stretch is greater, which causes both the edges of the belt to bend inwards.
Such a belt deformation in the image forming apparatus causes further problems.
In a structure in which a detecting mark is provided at one place of the inner surface of the belt and a sensor is provided at a position opposite to the detection mark, when the edges in the width direction of the belt bend inwards, the transit position of the detection mark gets shifted from the position opposite to the sensor. As a result, the detection timing goes off or the detection cannot be carried out.
When a belt suspended by a plurality of rollers is driven, in order to prevent biasing of the belt toward an edge side, a stopper is provided, which protrudes into the inner surface of both the edges in the width direction of the belt. In a belt in which the stopper is provided, when both the edges in the width direction bend inwards, instead of touching the ends of the roller, the stopper collide with the corners of the rollers or climb on to the side surfaces of the roller. This leads to a malfunction in the driving of the belt.
Moreover, the affixed material S is the scale for detecting the movement speed. When driving the belt, in a structure in which a sensor provided at a position opposite to the scale is used to carry out the belt movement speed detection, the following problem may arise. As shown in FIG. 16, as the angle of the scale becomes tilted with respect to a sensor 12, precision in the movement speed detection deteriorates and in some cases the detection cannot be carried out. In a structure in which various controls are carried out based on a result of the detection, malfunction occurs in those controls.
However, the tilt at the boundary d, which is caused due to the difference in the amount of stretch of the belt where the affixed material S is not provided and the part where the affixed material S is provided, is not necessarily confined to the edges in the width direction of the belt. For instance, when an affixed material is disposed along the center in the width direction of the belt, the stretch at the center of the belt is less while the stretch at both the edges is more, and the tilt occurs at the boundary. According to the deformation in the belt V, some unexpected malfunction may occur.